Methods for recovering argon

ABSTRACT

Methods for recovering argon from a natural gas stream being purified are disclosed. The argon will be separated from the natural gas with helium and other impurities and can be recovered as liquids from the flash separation step of purifying the helium. The argon that is separated from the helium can be rectified to remove further impurities from the argon which can be recovered for storage or other uses.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from International Application Serial No. PCT/US2007/087341 filed 13 Dec. 2007 (published as WO 2008/076782 A2, with publication date 26 Jun. 2008), which claims priority from U.S. Provisional Patent Application Ser. No. 60/875,509, filed Dec. 18, 2006.

BACKGROUND OF THE INVENTION

Current cosmological models predict the existence of “dark matter” to explain the observed expansion of the universe. Dark matter is so called because it hypothetically lacks the property of emissivity of electromagnetic radiation while still having substantial mass. Consequently, one shouldn't be able to “see” it as one can see normal matter reacting to electromagnetic radiation, but one should be able to detect it by its interactions with normal matter.

Many cosmological models predict that dark matter should be prevalent in a form called WIMPs, or “Weakly Interacting Massive Particles”. The large mass of these particles should make collisions with similar large atoms of normal matter observable, and the physics have been derived to quantify what these interactions should look like. As a result, it appears that argon forms the perfect collision target for WIMPs. Several investigators have designed a series of WIMP detectors using liquid Ar to capture the collisions and distinguish them from other radioactive phenomena. See “Measurement of the Specific Activity of ³⁹Ar in Natural Argon”, http://www.arxiv.org/pdf/astro-ph/0603131, 6 Mar. 2006.

A problem exists with the argon source, however, which has led some to seek new sources. The collision between a WIMP and the normal Ar isotope, ⁴⁰Ar, can yield ³⁹Ar as an artifact. Normal atmospheric argon contains a low level of ³⁹Ar (1 part in 1 million billion) due to the effects of solar radiation, so this background level creates “noise” in the detector signal. Argon from underground sources should in principle be free from ³⁹Ar and thus would constitute a more sensitive detector for WIMP. See “A Dating Method with ³⁹Ar”, H. H. Loosli, Earth and Planetary Science Letters, 63 (1983) 51-62.

Argon is known to exist in natural gases around the world. See “Helium—Its Relationship to Geologic Systems and its Occurrence with the Natural Gases, Nitrogen, Carbon Dioxide, and Argon”, Claude A. Tongish, United States, Bureau of Mines Report of Investigations 8444 (1980).

Some analyses show argon at average levels around 0.05-0.1%. This level is quite low and would not be economically recoverable by conventional means, especially without concomitant recovery and purification of any contained hydrocarbons and helium. The present invention is to directed to a means to concentrate and recover argon economically from natural gas.

SUMMARY OF THE INVENTION

The present invention relates to a method for recovering argon from natural gas. The proposed method improves on earlier methods of trying to separate the argon from the bulk of the natural gas stream, thereby reducing the size of the stream to be treated, and allowing for recovery of the valuable components (hydrocarbons and helium) that result from the separation. The liquid off the primary feed separator is currently used as part of liquid nitrogen circuit in the helium plant, but can be replaced by imported liquid nitrogen.

Accordingly, there is disclosed a method for recovering argon from bottoms liquid from the flash separation of a helium gas mixture containing argon comprising subjecting the bottoms liquid to a rectification process.

Further, the present invention provides for a method for recovering argon present in a natural gas stream during the purification of a helium gas mixture withdrawn from the natural gas stream comprising recovering the liquid bottoms from the helium purification and subjecting the liquid bottoms to a rectification process specific for argon.

Accordingly, in one embodiment of the invention, there is disclosed a method for recovering argon from a natural gas stream comprising the steps:

a) directing a natural gas stream containing natural gas, hydrocarbons, crude helium and argon to a helium recovery unit; b) separating a gas stream containing helium and argon from the natural gas stream and directing the helium and argon to a helium purifier; c) purifying the helium; and d) recovering argon from the purified helium.

In a further embodiment of the present invention there is disclosed a method for recovering argon from a natural gas stream comprising the steps;

a) separating crude helium and argon from the natural gas stream; b) directing the separated gas stream to a first helium separation unit, wherein argon is separated from the gas stream and directed to an argon concentration unit; c) directing the helium from the first helium separation unit to an enriched helium separation unit; d) concentrating argon by removing nitrogen from the argon concentration unit; and e) recovering concentrated argon from the argon concentration unit.

In a further embodiment of the present invention, there is disclosed a method for recovering argon during the purification of a helium gas stream mixture separated from a natural gas stream comprising recovering the liquid bottoms from the purification of the helium and subjecting the liquid bottoms to a rectification process to recover argon.

The purification of helium is typically performed in a helium recovery unit which can be a cryogenic rectifier. Once purification is achieved the helium is directed to either a helium liquefier or to storage.

Argon will typically be present in the natural gas stream in an amount ranging from about 10 ppm to about 10000 ppm.

The rectification that is performed on the argon is designed to separate the argon from any further impurities originating in the natural gas stream and passing through to the helium separation processes. This impurity typically is nitrogen and the rectification processes are typically cryogenic rectification and selective adsorption.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a schematic representation of the process whereby argon is recovered from a helium purification process.

DETAILED DESCRIPTION OF THE INVENTION

The FIGURE is a schematic representation of a crude helium stream 1 which has been recovered from a natural gas prior to the natural gas being added to a pipeline. The crude helium stream contains impurities such as nitrogen, methane, argon and other noble gases (see Table 1).

TABLE 1 Typical composition of crude helium extracted from the National Helium Reserve. Component Concentration by Volume He 77.3% N₂ 20.3% CH₄  1.6% H₂ 8000 ppm  Ar 680 ppm CO₂ 110 ppm

This stream is pre-treated to remove hydrogen and carbon dioxide, and then fed into the primary feed separator A where the crude helium is flash separated by being cooled to below about −300° F. The flashed helium which is about 95 to 98% by volume helium leaves the primary feed separator A through line 2 and enters the enriched helium separator B.

The enriched helium separator B is also a flash separator where the further purified helium is further purified and is at a lower temperature below about −315° F. than the primary feed separator A. This further purified helium can be recovered through line 6 for storage purposes. The liquids leaving the enriched helium separator B are primarily nitrogen and small amounts of other gases (Table 2) and exit through line 7 to connect with line 4 nitrogen discharge from concentrator C where the nitrogen can be stored for other uses or discharged to the atmosphere.

TABLE 2 Typical composition of rejected liquid from helium purification. Component Concentration by Volume He 0.5% N₂ 96.0%  CH₄ 3.5% H₂ 0 ppm Ar 2800 ppm   CO₂ 0 ppm

The bottoms from the primary feed separator A leave through line 3 and are directed to the ⁴⁰Ar concentrator C. The concentrator is designed to rectify the liquids mixture from the bottoms liquid from the primary feed separator A which comprise nitrogen, methane, argon and other noble gases. The rectification can be performed by a variety of techniques, including cryogenic rectification or adsorptive separation depending upon the concentration of the argon and other constituents in the bottoms liquid.

The rectified argon will leave the concentrator C through line 5 where it can be directed to storage or to an on-site usage as necessary. The nitrogen present in the bottoms liquid is also recovered through line 4 and can be directed to storage or discharged into the atmosphere.

While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention. 

1. A method for recovering argon from a natural gas stream comprising the steps: a) directing a natural gas stream containing natural gas, hydrocarbons, crude helium and argon to a helium recovery unit; b) separating a gas stream containing helium and argon from said natural gas stream and directing said helium and argon to a helium purifier; c) purifying said helium; and d) recovering argon from said purified helium.
 2. The method as claimed in claim 1 wherein said helium recovery unit is a cryogenic rectifier.
 3. The method as claimed in claim 2 wherein argon is present in an amount ranging from 10 ppm to about 10000 ppm.
 4. The method as claimed in claim 1 wherein said purified helium is directed to a helium liquefier or storage.
 5. The method as claimed in claim 1 wherein said argon is recovered by rectification.
 6. The method as claimed in claim 5 wherein said rectification is selected from the group consisting of cryogenic rectification and selective adsorption.
 7. A method for recovering argon from a natural gas stream comprising the steps; a) separating crude helium and argon from said natural gas stream; b) directing said separated gas stream to a first helium separation unit, wherein argon is separated from said gas stream and directed to an argon concentration unit; c) directing said helium from said first helium separation unit to an enriched helium separation unit; d) concentrating argon by removing nitrogen from said argon concentration unit; and e) recovering concentrated argon from said argon concentration unit.
 8. The method as claimed in claim 7 wherein said helium recovery unit is a cryogenic rectifier.
 9. The method as claimed in claim 8 wherein argon is present in an amount ranging from 10 ppm to about 10000 ppm.
 10. The method as claimed in claim 7 wherein said helium is withdrawn from said enriched helium separation unit and directed to a helium liquefier or storage.
 11. The method as claimed in claim 7 wherein said argon concentration unit is a rectification unit.
 12. The method as claimed in claim 11 wherein said rectification unit is a cryogenic rectification unit.
 13. The method as claimed in claim 11 wherein said argon concentration unit is an adsorptive separation unit.
 14. A method for recovering argon during the purification of a helium gas stream mixture separated from a natural gas stream comprising recovering the liquid bottoms from the purification of said helium and subjecting said liquid bottoms to a rectification process to recover argon.
 15. The method as claimed in claim 14 wherein said purification of helium is performed in a helium recovery unit.
 16. The method as claimed in claim 14 wherein said helium recovery unit is a cryogenic rectifier.
 17. The method as claimed in claim 16 wherein argon is present in an amount ranging from 10 ppm to about 10000 ppm.
 18. The method as claimed in claim 14 wherein said liquid bottoms contain argon and nitrogen.
 19. The method as claimed in claim 5 wherein said rectification is selected from the group consisting of cryogenic rectification and selective adsorption. 